Thin film emitter-absorber apparatus and methods

ABSTRACT

Methods and apparatus for providing a tunable absorption-emission band in a wavelength selective device are disclosed. A device for selectively absorbing incident electromagnetic radiation includes an electrically conductive surface layer including an arrangement of multiple surface elements. The surface layer is disposed at a nonzero height above a continuous electrically conductive layer. An electrically isolating intermediate layer defines a first surface that is in communication with the electrically conductive surface layer. The continuous electrically conductive backing layer is provided in communication with a second surface of the electrically isolating intermediate layer. When combined with an infrared source, the wavelength selective device emits infrared radiation in at least one narrow band determined by a resonance of the device. In some embodiments, the device includes a control feature that allows the resonance to be selectively modified. The device has broad applications including gas detection devices and infrared imaging.

RELATED APPLICATIONS

This application is claims the benefit of priority under 35 U.S.C. §119from U.S. Provisional Application Ser. No. 60/749,468, filed on Dec. 12,2005, the contents of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates generally to reflector andemitter-absorber structures, and more particularly to thin filmreflector and emitter-absorber structures formed using multipleconductive elements over a ground plane.

BACKGROUND OF THE INVENTION

Frequency selective surfaces can be provided to selectively reduce orenhance reflections from incident electromagnetic radiation. Suchsurfaces are often employed in signature management applications toreduce radar returns. These applications are typically employed withinthe radio frequency portion of the electromagnetic spectrum.

As modern radar systems are often equipped with different and evenmultiple frequency bands, such signature management surfaces arepreferably broad band, reducing reflections over a broad portion of thespectrum. Examples of known frequency selective surfaces providing sucha response include one or more than one dielectric layers, which may bedisposed above a ground plane. Thickness of the dielectric layerscombined with the selected material properties reduce reflectedradiation. The thickness of one or more of the layers is a predominantdesign criteria and is often on the order of one quarter wavelength.Unfortunately, such structures can be complicated and relatively thick,depending upon the selected dielectric materials and wavelength ofoperation, particularly since multiple layers are often employed.

The use of multiple frequency selective surfaces disposed above a groundplane, for radio frequency applications, is described in U.S. Pat. No.6,538,596 to Gilbert. The frequency selective surfaces can includeconductive materials in a geometric pattern with a spacing of themultiple frequency selective surface layers, which can be closer than aquarter wave. However, Gilbert seems to rely on the multiple frequencyselective surfaces providing a virtual continuous quarter wavelengtheffect. Such a quarter wavelength effect results in a canceling of thefields at the surface of the structure. Thus, although individual layersmay be spaced at less than one-quarter wavelength (e.g., λ/12 or λ/16),Gilbert relies on macroscopic (far field) superposition of resonancesfrom three of four sheets, such that the resulting structure thicknesswill be on the order of one-quarter wavelength.

SUMMARY OF THE INVENTION

What is needed is a simple, thin, wavelength selective surface capableof providing a tunable reflection or absorption-emission band.Preferably, the location of the reflection or absorption-emission bandas well as its bandwidth can be tuned.

Various embodiments of the present invention provide an apparatus andmethod for providing a tunable absorption-emission band in a highlyreflective wavelength selective surface. An array of surface elementsare defined in an electrically conductive layer disposed above acontinuous electrically conductive layer, or ground plane.

In a first aspect, the invention relates to a tunable device forselectively coupling electromagnetic radiation. The tunable deviceincludes a first electrically conductive layer having a group ofdiscrete surface elements. The tunable device also includes anelectrically insulating intermediate layer defining a first surface incommunication with the electrically conductive surface layer and asecond, continuous electrically conductive layer in communication with asecond surface of the electrically insulating intermediate layer. Aterminal is included in electrical communication with at least one ofthe first electrically conductive layer, the electrically insulatingintermediate layer, and the second continuous, electrically conductivelayer. The group of discrete surface elements resonantly couples atleast a portion of the electromagnetic radiation with respect to thecontinuous electrically conductive layer.

In another aspect, the invention relates to a tunable infrared (IR)emitter. The tunable IR emitter includes a first electrically conductivelayer including a group of discrete surface elements, an electricallyinsulating intermediate layer defining a first surface in communicationwith the electrically conductive surface layer, and a second, continuouselectrically conductive layer in communication with a second surface ofthe electrically insulating intermediate layer. The tunable IR devicealso includes an IR source in thermal communication with at least one ofthe first electrically conductive layer, the electrically insulatinglayer and the second, continuous electrically conductive layer. The IRsource generates broadband infrared radiation. The group of discretesurface elements electromagnetically couples at least a portion of thebroadband infrared radiation to produce a tuned, narrowband IR emission.

In another aspect, the invention relates to controllable wavelengthselective device. The controllable device includes a first electricallyconductive layer including a group of discrete surface elements, anelectrically insulating intermediate layer defining a first surface incommunication with the electrically conductive surface layer, and asecond, continuous electrically conductive layer in communication with asecond surface of the electrically insulating intermediate layer. Atleast one of the first electrically conductive layer, the electricallyinsulating intermediate layer, and the second electrically conductivelayer provides an externally controllable electrical conductivity.

In yet another aspect, the invention relates to a method ofmanufacturing a wavelength selective device. The method of manufacturingincludes forming a continuous, electrically thin conductive ground layeron a substrate. An electrically thin insulating layer is applied to atop surface of the ground layer. An electrically thin outer conductivelayer is formed on the electrically thin insulating layer. Theelectrically thin outer conductive layer includes a plurality ofdiscrete surface elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 shows a top perspective view of one embodiment of a wavelengthselective surface having a rectangular array of electrically conductivesurface elements.

FIG. 2 shows a top planar view of the wavelength selective surface ofFIG. 1.

FIG. 3 shows a top planar view of another embodiment of a wavelengthselective surface in accordance with the principles of the presentinvention having a hexagonal array of electrically conductive squaresurface elements.

FIG. 4 shows a top planar view of another embodiment of a wavelengthselective surface having two resonances.

FIG. 5 shows a top planar view of an alternative embodiment of the dualwavelength device of FIG. 4.

FIG. 6 shows a top perspective view of an alternative embodiment of awavelength selective surface having apertures defined in an electricallyconductive surface layer.

FIG. 7A shows a cross-sectional elevation view of the wavelengthselective surface of FIG. 1 taken along A-A.

FIG. 7B shows a cross-sectional elevation view of the wavelengthselective surface of FIG. 6 taken along B-B.

FIG. 7C shows a cross-sectional elevation view of an alternativeembodiment of a wavelength selective surface having a secondintermediate layer.

FIG. 8A shows a cross-sectional elevation view of an alternativeembodiment of a wavelength selective surface having an over layercovering electrically conductive surface elements.

FIG. 8B shows a cross-sectional elevation view of an alternativeembodiment of a wavelength selective surface having an over layercovering an electrically conductive surface layer and apertures definedtherein.

FIG. 9A shows in graphical form, an exemplaryreflectivity-versus-wavelength response of a narrowband wavelengthselective surface constructed in accordance with the principles of thepresent invention.

FIG. 9B shows in graphical form an exemplaryreflectivity-versus-wavelength response of a dual resonance deviceconstructed in accordance with the principles of the present invention.

FIG. 9C shows in graphical form, an exemplaryreflectivity-versus-wavelength response of a wideband wavelengthselective surface constructed in accordance with the principles of thepresent invention.

FIG. 10 shows in graphical form an exemplaryemissivity-versus-wavelength response of different wavelength selectivedevices constructed in accordance with the principles of the presentinvention.

FIG. 11 is a cross-sectional elevation of an embodiment of the presentinvention packaged in a TO-8 windowed can.

FIG. 12 is a plan view of an embodiment of the present invention formedin a serpentine ribbon.

FIG. 13 is an exemplary bridge drive circuit for a wavelength selectivesurface constructed in accordance with the present invention.

FIG. 14A shows in schematic form an embodiment of a substance detectorincluding a single element source and detector with a spherical mirror.

FIG. 14B shows in schematic form an alternative embodiment of asubstance detector including separate source and detector elements usinga reflective surface.

FIG. 15A is a side elevation of one embodiment of a wavelength selectivesurface having a controllable conductivity over layer.

FIG. 15B is a top perspective diagram of an embodiment of a wavelengthselective surface having a controllable conductivity over layer.

FIG. 16 is a plan view of an embodiment of a pixel incorporatingwavelength selective devices.

FIG. 17 is a schematic plan view of a matrix display incorporating thepixels of FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description of preferred embodiments of the invention follows.

An exemplary embodiment of a wavelength selective surface 10 is shown inFIG. 1. The wavelength selective surface 10 includes at least threedistinguishable layers. The first layer is an electrically conductiveouter or surface layer 12 including an arrangement of surface elements20. The surface elements 20 of the outer layer 12 are disposed at aheight above an inner layer including a continuous electricallyconductive sheet, or ground layer 14. The arrangement of surfaceelements 20 and ground layer 14 is separated by an intermediate layer 16disposed therebetween. At least one function of the intermediate layer16 is to maintain a physical separation between the arrangement ofsurface elements 20 and the ground layer 14. The intermediate layer 16also provides electrical isolation between the two electricallyconductive layers 12, 14.

In operation, wavelength selective surface 10 is exposed to incidentelectromagnetic radiation 22. A variable portion of the incidentradiation 22 is coupled to the wavelength selective surface 10. Thelevel of coupling depends at least in part upon the wavelength of theincident radiation 22 and a resonant wavelength of the wavelengthselective surface 10, as determined by related design parameters.Radiation coupled to the wavelength selective surface 10 can also bereferred to as absorbed radiation. At other non-resonant wavelengths, asubstantial portion of the incident radiation is reflected 24.

In more detail, the electrically conductive surface layer 12 includesmultiple discrete surface features, such as the electrically conductivesurface elements 20 arranged in a pattern along a surface 18 of theintermediate layer 16. The discrete nature of the arrangement of surfacefeatures 20 requires that individual surface elements 20 are isolatedfrom each other. This also precludes interconnection of two or moreindividual surface elements 20 by electrically conducting paths. Two ormore individual surface elements which are connected electrically form acomposite surface element which gives rise to a new resonance.

The electrically conductive surface layer 12 including an arrangement ofsurface elements 20 is typically flat, having a smallest dimension,height, measured perpendicular to the intermediate layer surface 18. Ingeneral, each surface element 20 defines a surface shape and a height orthickness measured perpendicular to the intermediate layer surface 18.In general, the surface shape can be any closed shape, such as closedcurves, regular polygons, irregular polygons, star-shapes having threeor more legs, and other closed structures bounded by piecewisecontinuous surfaces including one or more curves and lines. In someembodiments, the surface shapes can include annular features, such asring shaped patch with an open center region. More generally, theannular features have an outer perimeter defining the outer shape of thepatch and an inner perimeter defining the shape of the open inner regionof the patch. Each of the outer an inner perimeters can have a similarshape, as in the ring structure, or a different shape. Shapes of theinner and outer perimeters can include any of the closed shapes listedabove (e.g., a round patch with a square open center).

The shapes can be selected to provide a resonant response having apreferred polarization. For example, surface features having anelongated shape provide a resonant response that is more pronounced in apolarization that is related to the orientation of the elongated shape.Thus, an array of vertically aligned narrow rectangles produces aresponse having a vertically aligned linear polarization. In general,preferred polarizations can be linear, elliptical, and circular.

Each of the electrically conductive surface elements 20 is formed withan electrically conductive material. Such conductive materials includeordinary metallic conductors, such as aluminum, copper, gold, silver,platinum, manganese, iron, nickel, tin, lead, and zinc; as well ascombinations of one or more metals in the form of a metallic alloy, suchas steel, and ceramic conductors such as indium tin oxide and titaniumnitride. Alternatively or in addition, conductive materials used information of the surface elements 20 include semiconductors. Preferably,the semiconductors are electrically conductive. Exemplary semiconductormaterials include: silicon and germanium; compound semiconductors suchas silicon carbide, gallium-arsenide and indium-phosphide; and alloyssuch as silicon-germanium and aluminum-gallium-arsenide. Electricallyconductive semiconductors are typically doped with one or moreimpurities in order to provide good electrical conductivity. Similarly,the ground layer 14 can include one or more electrically conductivematerials, such as those described herein.

The intermediate layer 16 can be formed from an electrically insulativematerial, such as a dielectric providing electrical isolation betweenthe arrangement of surface elements 20 and the ground layer 14. Someexamples of dielectric materials include silicon dioxide (SiO₂); alumina(Al₂O₃); aluminum oxynitride; silicon nitride (Si₃N₄). Other exemplarydielectrics include polymers, rubbers, silicone rubbers, cellulosematerials, ceramics, glass, and crystals. Dielectric materials alsoinclude: semiconductors, such as silicon and germanium; compoundsemiconductors such as silicon carbide, gallium-arsenide andindium-phosphide; and alloys such as silicon-germanium andaluminum-gallium-arsenide; and combinations thereof. As dielectricmaterials tend to concentrate an electric field within themselves, anintermediate dielectric layer 16 will do the same, concentrating aninduced electric field between each of the surface elements 20 and aproximal region of the ground layer 14. Beneficially, such concentrationof the electric-field tends to enhance electromagnetic coupling of thearrangement of surface elements 12 to the ground layer 14.

Dielectric materials can be characterized by parameters indicative oftheir physical properties, such as the real and imaginary portions ofthe index of refraction, often referred to as “n” and “k.” Althoughconstant values of these parameters n, k can be used to obtain anestimate of the material's performance, these parameters are typicallywavelength dependent for physically realizable materials. In someembodiments, the intermediate layer 16 includes a so-called high-kmaterial. Examples of such materials include oxides, which can have kvalues ranging from 0.001 up to 10.

The arrangement of surface elements 20 can be configured in a preferredarrangement, or array on the intermediate layer surface 18. Referringnow to FIG. 2, the wavelength selective surface 10 includes an exemplaryarray of flattened, electrically conductive surface elements 20.Multiple surface elements 20 are arranged in a square grid along theintermediate layer surface 18. A square grid or matrix arrangement is anexample of a regular array, meaning that spacing between adjacentsurface elements 20 is substantially uniform. Other examples of regulararrays, or grids include hexagonal grids, triangular grids, obliquegrids, centered rectangular grids, and Archimedean grids. In someembodiments, the arrays can be irregular and even random. Each of theindividual elements 20 can have substantially the same shape, such asthe circular shape shown.

Although flattened elements are shown and described, other shapes arepossible. For example, each of the multiple surface elements 20 can havenon-flat profile with respect to the intermediate layer surface 18, suchas a parallelepiped, a cube, a dome, a pyramid, a trapezoid, or moregenerally any other shape. One major advantage of the present inventionover other prior art surfaces is a relaxation of the fabricationtolerances. The high field region resides underneath each of themultiple surface elements 20, between the surface element 20 and acorresponding region of the ground layer 14.

In more detail, each of the circular elements 20 has a respectivediameter D. In the exemplary square grid, each of the circular elements20 is separated from its four immediately adjacent surface elements 20by a uniform grid spacing A measured center-to-center. An alternativeembodiment of another wavelength selective surface 40 including ahexagonal arrangement, or array of surface elements 42 is shown in FIG.3. Each of the discrete surface elements includes a square surfaceelement 44 having a side dimension D′. Center-to-center spacing betweenimmediately adjacent elements 44 of the hexagonal array 42 is about A′.For operation in the infrared portion of the electromagnetic spectrum, Dwill generally be between about 0.5 microns for near infrared and 50microns for the far infrared and terahertz, understanding that any suchlimits are not firm and will very depending upon such factors as n, k,and the thickness of layers.

Array spacing A can be as small as desired, as long as the surfaceelements 20 do not touch each other. Thus, a minimum spacing will dependto some extent on the dimensions of the surface feature 20. Namely, theminimum spacing must be greater than the largest diameter of the surfaceelements (i.e., A>D). The surface elements can be separated as far asdesired, although absorption response suffers from increased gridspacing as the fraction of the total surface covered by surface elementsfalls below 10%.

In some embodiments, more than one arrangement of uniform-sized featuresare provided along the same outer surface layer of a wavelengthselective surface. Shown in FIG. 4 is a plan view of one such device 100having two different arrangements of electrically conductive features102 a, 102 b (generally 102) disposed along the same surface. The firstarrangement 102 a includes a triangular array, or grid, of uniform-sizedcircular patches 104 a, each having a diameter D₁ and separated from itsnearest neighbors by a uniform grid spacing A. Similarly, the secondarrangement 102 b includes a triangular grid of uniform-sized circularpatches 104 b, each having a diameter D₂ and separated from its nearestneighbors by a uniform grid spacing A. Visible between the circularpatches 104 a, 104 b is an outer surface 18 of the intermediate layer.Each of the arrangements 102 a, 102 b occupies a respective,non-overlapping region 106 a, 106 b of the intermediate layer surface18. Except for there being two different arrangements 102 a, 102 b onthe same surface 18, the device 100 is substantially similar to theother wavelength selective devices described hereinabove. That is, thedevice 100 also includes a ground plane 14 (not visible in this view)and an intermediate insulating layer 16 disposed between the groundplane 14 and a bottom surface of the circular patches 104 a, 104 b.

Continuing with this illustrative example, each of the differentarrangements 102 a, 102 b is distinguished from the other by therespective diameters of the different circular patches 104 a, 104 b(i.e., D₂>D₁). Other design attributes including the shape (i.e.,circular), the grid format (i.e., triangular), and the grid spacing ofthe two arrangements 102 a, 102 b are the substantially the same. Othervariations of a multi-resonant device are possible with two or moredifferent surface arrangements that differ from each other according toone or more of: shape; size; grid format; spacing; and choice ofmaterials. Size includes thickness of each of the multiple layers 14,16, 102 of the device 100. Different materials can also be used in oneor more of the regions 106 a, 106 b. For example, an arrangement of goldcircular patches 102 a in one region 106 a and an arrangement ofaluminum circular patches 102 b in another region 106 b.

In operation, each of the different regions 106 a, 106 b willrespectively contribute to a different resonance from the samewavelength selective device 100. Thus, one device can be configured toselectively provide a resonant response to incident electromagneticradiation within more than one spectral regions. Such features arebeneficial in IR applications in which the device 100 provides resonantemission peaks in more than one IR band. Thus, a first resonant peak canbe provided within a 3-5 μm IR band, while a second resonant peak can besimultaneously provided within a 7-14 μm IR band, enabling the samedevice to be simultaneously visible to IR detectors operating in eitherof the two IR bands.

In some embodiments, the different arrangements 102 a′ and 102 b′ canoverlap within at least a portion of the same region. An exemplaryembodiment is shown in FIG. 5 having a substantially complete overlap,in which a first arrangement 102 a′ includes a triangular grid ofuniform-sized circular patches 104 a′ of a first diameter D₁, interposedwithin the same region with a second arrangement 102 b′ including atriangular grid of uniform-sized circular patches 104 b′ of a seconddiameter D₂. Each arrangement 102 a′, 102 b′ has a grid spacing of A.When exposed to incident electromagnetic radiation, device 100′ willproduce more than one resonant features, with each resonant featurecorresponding to a respective one of the different arrangements 102 a′,102 b′. As with the previous example, one or more of the parametersincluding: shape; size; grid format; spacing; and choice of materialscan be varied between the different arrangements 102 a′, 102 b′.

In yet other embodiments (not shown), devices similar to those describedabove in relation to FIG. 4 and FIG. 5 are formed having a complementarysurface. Thus, a single device includes two or more differentarrangements of through holes formed in an electrically conductive layerabove and isolated from a common ground layer. One or more of thethrough-hole size, shape, grid format, grid spacing, thickness, andmaterials can be varied to distinguish the two or more differentarrangements. Once again, the resulting device exhibits at least onerespective resonant feature for each of the two or more differentarrangements.

An exemplary embodiment of an alternative family of wavelength selectivesurfaces 30 is shown in FIG. 6. The alternative wavelength selectivesurfaces 30 also include an intermediate layer 16 stacked above a groundlayer 14; however, an electrically conductive surface 32 layer includesa complementary feature 34. The complementary feature 34 includes theelectrically conductive layer 32 defining an arrangement of throughapertures, holes, or perforations.

The electrically conductive layer 32 is generally formed having auniform thickness. The arrangement of through apertures 34 includesmultiple individual through apertures 36, each exposing a respectivesurface region 38 of the intermediate layer 16. Each of the throughapertures 36 forms a respective shape bounded by a closed perimeterformed within the conductive layer 32. Shapes of each through aperture36 include any of the shapes described above in reference to theelectrically conductive surface elements 20 (FIG. 1), 44 (FIG. 3).

Additionally, the through apertures 36 can be arranged according to anyof the configurations described above in reference to the electricallyconductive surface elements 20, 44. This includes a square grid, arectangular grid, an oblique grid, a centered rectangular grid, atriangular grid, a hexagonal grid, and random grids. Thus, any of thepossible arrangements of surface elements 36 and corresponding exposedregions of the intermediate layer surface 18 can be duplicated in acomplementary sense in that the surface elements 20 are replaced bythrough apertures 36 and the exposed regions of the intermediate layersurface 18 are replaced by the electrically conductive layer 32.

A cross-sectional elevation view of the wavelength selective surface 10is shown in FIG. 7A. The electrically conductive ground layer 14 has asubstantially uniform thickness H_(G). The intermediate layer 16 has asubstantially uniform thickness H_(D), and each of the individualsurface elements 20 has a substantially uniform thickness H_(P). Thedifferent layers 12, 14, 16 can be stacked without gaps therebetween,such that a total thickness H_(T) of the resulting wavelength selectivesurface 10 is substantially equivalent to the sum of the thicknesses ofeach of the three individual layers 14, 16, 12 (i.e.,H_(T)=H_(G)+H_(D)+H_(p)). A cross-sectional elevation view of thecomplementary wavelength selective surface 30 is shown in FIG. 7B andincluding a similar arrangement of the three layers 14, 16, 32.

In some embodiments, the intermediate insulating layer has a non-uniformthickness with respect to the ground layer. For example, theintermediate layer may have a first thickness H_(D) under each of thediscrete conducting surface elements and a different thickness, orheight at regions not covered by the surface elements. It is importantthat a sufficient layer of insulating material be provided under each ofthe surface elements to maintain a design separation and to provideisolation between the surface elements and the ground layer. In at leastone example, the insulating material can be substantially removed at allregions except those immediately underneath the surface elements. Inother embodiments, the insulating layer can include variations, such asa taper between surface elements. At least one benefit of the inventivedesign is a relaxation of design tolerances that results in asimplification of fabrication of the devices.

The thickness chosen for each of the respective layers 12, 32, 16, 14(H_(P), H_(D), H_(G)) can be independently varied for variousembodiments of the wavelength selective surfaces 10, 30. For example,the ground plane 14 can be formed relatively thick and rigid to providea support structure for the intermediate and surface layers 16, 12, 32.Alternatively, the ground plane 14 can be formed as a thin layer, aslong as a thin ground plane 14 forms a substantially continuouselectrically conducting layer of material providing the continuousground. Preferably, the round plane 14 is at least as thick as one skindepth within the spectral region of interest. Similarly, in differentembodiments of the wavelength selective surfaces 10, 30, the respectivesurface layer 12, 32 can be formed with a thickness H_(P) ranging fromrelatively thin to relatively thick. In a relatively thin embodiment,the surface layer thickness H_(P) can be a minimum thickness requiredjust to render the intermediate layer surface 18 opaque. Preferably, thesurface layer 12, 32 is at least as thick as one skin depth within thespectral region of interest.

Likewise, the intermediate layer thickness H_(D) can be formed as thinas desired, as long as electrical isolation is maintained between theouter and inner electrically conducting layers 12, 32, 14. The minimumthickness can also be determined to prevent electrical arcing betweenthe isolated conducting layers under the highest anticipated inducedelectric fields. Alternatively, the intermediate layer thickness H_(D)can be formed relatively thick. The concept of thickness can be definedrelative to an electromagnetic wavelength ‘λ_(c)’ of operation, orresonance wavelength. For example, the intermediate layer thicknessH_(D) can be selected between about 0.01λ_(c) in a relatively thinembodiment to about 0.5λ_(c) in a relatively thick embodiment.

Referring to FIG. 7C, a cross sectional view of a wavelength selectivedevice 38 includes an arrangement of surface features 20 disposed overground plane 14, with an intermediate insulating layer 16 disposedbetween the surface features 20 and the ground plane 14. The device 38also includes a second intermediate layer 39 disposed between a topsurface 18 of the insulating layer and a bottom surface of the surfacefeatures 20. The second layer 39 is also an insulating material, suchthat the individual surface features 20 remain discrete and electricallyisolated from each other with respect to a non time-varying electricalstimulus. For example, the second intermediate layer 39 can be formedfrom a dielectric material chosen to have material properties n, kdifferent than the material properties of the first intermediate layer16. Any dielectric material can be used including any of the dielectricmaterials described herein. Alternatively or in addition, the secondintermediate layer 39 can be formed from a semiconductor material. Anysemiconductor can be used, including those semiconductor andsemiconductor compounds described herein, provided that thesemiconductor includes an electrically insulating mode. More generally,a fourth layer having physical properties described above in relation tothe second intermediate layer 39 can be provided between any of thethree layers 14, 16, 20 of the device 38.

The wavelength selective surfaces 10, 30 can be formed using standardsemiconductor fabrication techniques. Thin devices can be obtained usingstandard fabrication techniques on a typical semiconductor substrate,followed by a release step, which the thin device is released from thesubstrate. One such technique is referred to as back-side etching, inwhich a sacrificial layer is removed underneath the device formed uponthe semiconductor substrate. Removal of the sacrificial layer releases athin-film device from the substrate.

Alternatively or in addition, the wavelength selective surfaces 10, 30can be formed using thin film techniques including vacuum deposition,chemical vapor deposition, and sputtering. In some embodiments, theconductive surface layer 12, 44 can be formed using printing techniques.The surface features can be formed by providing a continuouselectrically conductive surface layer and then removing regions of thesurface layer to form the surface features. Regions can be formed usingstandard physical or chemical etching techniques. Alternatively or inaddition, the surface features can be formed by laser ablation, removingselected regions of the conductive material from the surface, or bynano-imprinting or stamping, or other fabrication methods known to thoseskilled in the art.

Referring to FIG. 8A a cross-sectional elevation view of an alternativeembodiment of a wavelength selective surface 50 is shown having an overlayer 52. Similar to the embodiments described above, the wavelengthselective surface 50 includes an electrically conductive outer layer 12having an arrangement of surface elements 20 (FIG. 1) disposed at aheight above a ground layer 14 and separated therefrom by anintermediate layer 16. The over layer 52 represents a fourth layer, orsuperstrate 52 provided on top of the electrically conductive surfacelayer 12.

The over layer 52 can be formed having a thickness H_(C1) measured fromthe intermediate layer surface 18. In some embodiments, the over layerthickness H_(C1) is greater than thickness of the surface elements 20(i.e., H_(C1)>H_(P)). The over layer 52 can be formed with varyingthickness to provide a planar external surface. Alternatively or inaddition, the over layer 52 can be formed with a uniform thickness,following a contour of the underlying electrically conductive surface12.

An over layering material 52 can be chosen to have selected physicalproperties (e.g., k, n) that allow at least a portion of incidentelectromagnetic radiation to penetrate into the over layer 52 and reactwith one or more of the layers 12, 14, and 16 below. In someembodiments, the overlying material 52 is optically transparent in thevicinity of the primary absorption wavelength, to pass substantially allof the incident electromagnetic radiation. For example, the overlyingmaterial 52 can be formed from a glass, a ceramic, a polymer, or asemiconductor. The overlaying material 52 can be applied using any oneor more of the fabrication techniques described above in relation to theother layers 12, 14, 16 in addition to painting and/or dipping.

In some embodiments, the over layer 52 provides a physical propertychosen to enhance performance of the wavelength selective device in anintended application. For example, the overlaying material 52 may haveone or more optical properties, such as absorption, refraction, andreflection. These properties can be used to advantageously modifyincident electromagnetic radiation. Such modifications include focusing,de-focusing, and filtering. Filters can include low-pass, high-pass,band pass, and band stop.

The overlaying material 52 can be protective in nature allowing thewavelength selective surface 50 to function, while providingenvironmental protection. For example, the overlaying material 52 canprotect the surface conductive layer 12 from corrosion and oxidation dueto exposure to moisture. Alternatively or in addition, the overlayingmaterial 52 can protect either of the exposed layers 12, 16 from erosiondue to a harsh (e.g., caustic) environment. Such harsh environmentsmight be encountered routinely when the wavelength selective surface isused in certain applications. At least one such application that wouldbenefit from a protective overlaying material 52 would be a marineapplication, in which a protective over layer 52 would protect theelectrically conductive layer 12 or 32 from corrosion.

In another embodiment shown in FIG. 8B, a wavelength selective surface60 includes an overlying material 62 applied over a conductive layer 32defining an arrangement of through apertures 34 (FIG. 6). The overlyingmaterial 62 can be applied with a maximum thickness H_(C2) measured fromthe intermediate layer surface 18 to be greater than the thickness ofthe conductive layer 32 (i.e., H_(C2)>H_(P)). The overlaying material 62again can provide a planar external surface or a contour surface.Accordingly, a wavelength selective surface 60 having apertures 34defined in an electrically conductive layer 32 is covered by anoverlying material 62. The performance and benefits of such a device aresimilar to those described above in relation to FIG. 8A.

Referring to FIG. 9A, an exemplary reflectivity versus wavelengthresponse curve 70 of a representative narrow-resonance response is shownin graphical form. The response curve 70 is achieved by exposing awavelength selective surface 10 (FIG. 1) constructed in accordance withthe principles of the present invention to incident electromagneticradiation 22 (FIG. 1) within a band including a resonance. As shown, thereflectivity to incident electromagnetic radiation varies according tothe curve 70 within the range of 0% to 100%. As the wavelength of theincident radiation 22 is varied from 2 to 20 microns, the reflectivitystarts at a relatively high value of about 75%, increases to a value ofover 85% at about 3 microns, reduces back to about 75% at about 3.5microns, and increases again to nearly 100% between about 3.5 and 7microns. Between 7 and 8 microns, the reflectivity response curve 70incurs a second and more pronounced dip 72 to less then 20%reflectivity. The second dip 72 is steep and narrow, corresponding toabsorption of incident electromagnetic radiation by the surface 10. Thereflectivity response curve 70 at wavelengths beyond about 8 micronsrises sharply back to more than 90% and remains above about 80% out toat least 20 microns. This range, from 2 to 20 microns, represents aportion of the electromagnetic spectrum including infrared radiation.

The second and much more pronounced dip 72 corresponds to a primaryresonance of the underlying wavelength selective surface 10. As a resultof this resonance, a substantial portion of the incident electromagneticenergy 22 is absorbed by the wavelength selective surface 10. A measureof the spectral width of the resonance response 70 can be determined asa width in terms of wavelength normalized to the resonant wavelength(i.e., Δλ/λ_(c) or dλ/λ_(c)). Preferably, this width is determined atfull-width-half-maximum (FWHM). For the exemplary curve, the width ofthe absorption band at FWHM is less than about 0.2 microns with anassociated resonance frequency of about 7 microns. This results in aspectral width, or dλ/λ_(c) of about 0.03. Generally, a dλ/λ_(c) valueof less than about 0.1 can be referred to as narrowband. Thus, theexemplary resonance is representative of a narrowband absorptionresponse.

Results supported by both computational analysis of modeled structuresand measurements suggest that the resonant wavelength associated withthe primary resonance response 72 corresponds to a maximum dimension ofthe electrically conductive surface elements (e.g., a diameter of acircular patch D, or a side length of a square patch D′). As thediameter of the surface elements is increased, the wavelength of theprimary absorption band 72 also increases. Conversely, as the diameterof the surface elements is decreased, the wavelength of the primaryabsorption band 72 also decreases.

The first, less pronounced dip 74 in reflectivity corresponds to asecondary absorption band of the underlying wavelength selective surface10. Results supported by both computational analysis of modeledstructures and measurements suggest that the wavelength associated withthe secondary absorption band 74 corresponds at least in part to acenter-to-center spacing of the multiple electrically conductive surfaceelements. As the spacing between surface elements 20 in the arrangementof surface elements 12 is reduced, the wavelength of the secondaryabsorption band 74 decreases. Conversely, as the spacing between thearrangement of surface elements 12 is increased, the wavelength of thesecondary absorption band 74 increases. The secondary absorption band 74is typically less pronounced than the primary absorption band 72 suchthat a change in reflectivity ΔR can be determined between the twoabsorption bands 74, 72. A difference in wavelength between the primaryand secondary absorption bands 72, 74 is shown as ΔW.

In general, the performance may be scaled to different wavelengthsaccording to the desired wavelength range of operation. Thus, by scalingthe design parameters of any of the wavelength selective surfaces asdescribed herein, resonant performance can be obtained within anydesired region of the electromagnetic spectrum. Resonant wavelengths canrange down to visible light and even beyond into the ultraviolet andX-ray. At the other end of the spectrum, the resonant wavelengths canrange into the terahertz band (e.g., wavelengths between about 1millimeter and 100 microns) and even up to radio frequency bands (e.g.,wavelengths on the order of centimeters to meters). Operation at theshortest wavelengths will be limited by available fabricationtechniques. Current techniques can easily achieve surface featuredimensions to the sub-micron level. It is conceivable that such surfacefeatures could be provided at the molecular level using currentlyavailable and emerging nanotechnologies. Examples of such techniques arereadily found within the field of micro-mechanical-electrical systems(MEMS).

An exemplary reflectivity versus wavelength response curve 110 is shownin FIG. 9B for a device 100 (FIG. 4) having more than one primaryresonances. In this example, a first resonance 112 a occurs at about 4.5μm and a second resonance 112 b occurs at about 9 μm. Also identified onthe graph are two different channels within the IR band. A first channel114 a extends from about 3 μm to about 5 μm; whereas, the second channel114 b extends from about 7 μm to about 14 μm. Advantageously, the firstresonance 112 a resides within the first IR channel 114 a and the secondresonance 112 b resides within the second IR channel 114 b. In otherembodiments of the present invention, one or more of the resonances 112a, 112 b can be selected through the proper choice of design parameters,to reside at a wavelength outside of a channel 114 a, 114 b.

Referring to FIG. 9C, an exemplary reflectivity versus wavelengthresponse curve 80 of a wide-resonance wavelength selective surface isshown in graphical form. This wideband response curve 80 can also beachieved with the wavelength selective surface 10 (FIG. 1) constructedin accordance with the principles of the present invention, but having adifferent selection of design parameters. Here, a primary absorptionband 82 occurs at about 8 microns, with wavelength range at FWHM ofabout 3 microns. This results in a spectral width Δλ/λ_(c) of about 0.4.A spectral width value Δλ/λ_(c) greater than 0.1 can be referred to asbroadband. Thus, the underlying wavelength selective surface 10 can alsobe referred to as a broadband structure.

One or more of the physical parameters of the wavelength selectivesurface 10 can be varied to control reflectivity and absorption-emissionresponse of a given wavelength selective surface. For example, thethickness of one or more layers (e.g., surface element thickness H_(p),dielectric layer thickness H_(D), and over layer thickness H_(C)) can bevaried. Alternatively or in addition, one or more of the materials ofeach of the different layers can be varied. For example, the dielectricmaterial can be substituted with another dielectric material having adifferent n and k values. The presence or absence of an over layer 52(FIG. 8A), as well as the particular material selected for the overlayer 52 can also be used to vary the reflectivity orabsorption-emission response of the wavelength selective surface.Similar performance changes may be achieved by changing the material ofthe ground plane, change the dimension D of the surface elements, or bychanging the shape of the surface elements.

In a first example, a wavelength selective surface includes anintermediate layer formed with various diameters of surface patches. Thewavelength selective surface includes a triangular array of roundaluminum patches placed over an aluminum film ground layer. The varioussurfaces are each formed with surface patches having a differentrespective diameter. A summary of results obtained for the differentpatch diameters is included in Table 1. In each of these exemplaryembodiments, the patch spacing between adjacent patch elements was about3.4 microns, and the thickness or depth of the individual patches and ofthe ground layer film were each about 0.1 micron. An intermediate,dielectric layer having thickness of about 0.2 microns was includedbetween the two aluminum layers. It is worth noting that the overallthickness of the wavelength selective surface is about 0.4 microns—avery thin material. The exemplary dielectric has an index of refractionof about 3.4. Table 1 includes wavelength values associated with theresulting primary absorptions. As shown, the resonant wavelengthincreases with increasing patch size.

TABLE 1 Primary Absorption Wavelength Versus Patch Diameter PatchDiameter Resonant Wavelength (λ_(c)) 1.25 μm 4.1 μm 1.75 μm 5.5 μm 2.38μm 7.5 μm 2.98 μm 9.5 μm

In another example, triangular arrays of circular patches having auniform array spacing of 3.4 microns and patch diameter of 1.7 micronsare used. A dielectric material provided between the outer conductinglayers is varied. As a result, the wavelength of the primary absorptionshifts. Results are included in Table 2.

TABLE 2 Resonance Versus Dielectric Material Dielectric materialResonant Wavelength (λc) Oxide 5.8 μm Nitride 6.8 μm Silicon 7.8 μm

Referring to FIG. 10, an exemplary emissivity versus wavelength curve120 a is shown within a portion of the IR spectrum for a devicefabricated in accordance with the present invention. When combined witha thermal source of radiation, wavelength selective surfaces accordingto the principles of the present invention produce a resonant responsein emissivity as determined at least in part to one or more physicalaspects of the underlying device.

As shown, the emissivity 120 a is relatively low (e.g., below about0.04) for wavelengths both below about 4 μm and above about 6 μm.However, at wavelengths between 4 μm and 6 μm a sharp rise in emissivityoccurs producing a peak emissivity 122 a corresponding to a resonantwavelength of the device. In the exemplary figure, the peak emissivity122 a is about 0.15 at a corresponding resonant frequency of about 4.5μm. As with reflectivity, a measure of the resonant response can bedefined by its selectivity determined as the spectral width at FWHMdivided by the resonant frequency (i.e., λc/λc). A selectivity value ofthe first resonant peak is about 0.1, for narrowband operation.

Also shown is a second curve 120 b having a different emissivity ofabout 0.06 at about 7.5 μm. Superimposed is a representative black bodycurve 124. Variation of one or more of the design parameters asdescribed herein can be used to choose the resonant wavelength 122 a,122 b. Thus, when the wavelength selective device or surface producingeither curve 120 a, 120 b is applied to a thermal source, such as afilamentary heater, the emissivity of the blackbody thermal source ismodified substantially to radiate only within a narrow band ofwavelengths corresponding to resonance frequency. Thus, a narrowband(i.e., λcλc<0.1) thermal source is possible combining the wavelengthselective device with a broadband thermal radiation source to produce asubstantially coherent IR source.

At least one important application for wavelength selective devicesaccording to principles of the present invention is in gas detectors. Asdescribed in U.S. Pat. No. 7,119,337, incorporated herein by referencein its entirety, a narrowband thermal source can be tuned to anabsorption band of a target gas. A sample of a substance, such as a gasis illuminated with the narrowband thermal source. A portion of theemitted spectrum is detected after propagating through the sample. Whenthe target gas is present, the detected radiation will be substantiallyless due to absorption by the gas.

Referring to FIG. 11, a thermal source 130 includes a narrowband IRsource 132 within an electrical device package 134. In an exemplaryembodiment, the IR source 132 is a horizontal thin film prepared inaccordance with the device of FIG. 1, including an arrangement ofuniform-sized electrically conductive surface patches above a groundplane separated by an intermediate thin-film layer of insulatingmaterial. The ground plane is provided with a finite conductivity havinga real resistive component. The thin film device 132 is suspended in abridge configuration between a pair of vertical support members 134 a,134 b. Electrical terminals 136 a, 136 b are used to inject anelectrical current into the ground plane of the emission device 132 toproduce thermal energy through a process referred to as Joule heating,or equivalently as I²R heating.

The device package 133 may include a sealed housing, such as a TO-8transistor used in standard process equipment, to isolate the IR source132 from the environment. The package 133 includes at least one window138 substantially aligned with an emission surface of the IR source 132,such that IR emissions can exit the package 133 to interact with theenvironment. The window 138 may include one or more optical propertiesincluding reflection, absorption, and transmission. In some embodiments,the device 130 includes a feature, such as the collar 135 shownproviding a smooth reflective surface disposed around the IR source 132and adapted to collect radiation emitted from the surface to selectivelydirect IR emissions within a preferred direction. Alternatively or inaddition, a reflective member 133 is provided on the floor of thepackage, underneath the suspended IR source 132 (e.g., on an interiorsurface of the header of the transistor can shown) to reflect emissionfrom a back side of the IR source 132 toward the window 138.Additionally, the package 133 includes one or more electrical leads 139a, 139 b that can be used to inject an electrical current to drive theIR source 132. More generally, the IR source 132 includes any of thethin film wavelength selective surfaces described herein combined with athin film thermal source—which can be the ground plane.

In some embodiments, a wavelength selective device, such as the IRsource 132 above, includes additional layers, including a differentrespective insulating layer on each surface of the ground layer. Eachinsulating layer can have a respective arrangement of electricallyconductive surface elements. Such a device is bidirectional in that itprovides a respective reflectivity-absorption and emission profile oneither side of the ground plane. A resonant performance of each of thedifferent sides is independently controllable according to selecteddesign parameters. In some embodiments, the design parameters of eachside of the device are substantially identical yielding similarresonances. Alternatively, the design parameters of each side of thedevice are substantially different yielding different resonances.

Referring to FIG. 12, an IR source 140 can include a first IR source 142a formed in a ribbon or filament configuration. The first filament 142 acan be formed in a serpentine shape, as shown, having electricalterminals 144 a, 144 b at either end. The electrical current can beapplied between the terminals 144 a, 144 b causing a resistive groundplane to heat.

A second filament 142 b can be provided within the same IR source 140.Preferably, the second filament 142 b is constructed similar to thefirst 142 a. In some embodiments, the second filament 142 b is used as adetector, detecting a reflected return of IR emissions from the firstfilament 142 a. In some embodiments, the second filament 142 b iscovered, or “blinded” by a screen 146. Thus, the second filament 142shielded by the screen 146 does not respond to received IR from outsidethe package, but is allowed to respond to other environmental anddevice-dependent effects, such as ambient temperature and long-termvariations in performance due to aging of the device. When formulatedfrom the same material, the second filament 142 b can be used as areference to compare response measured on the first filament 142 a.Thus, effects due to ambient temperature and long-term aging can beeffectively removed from measurements obtained from the first.

In general, drive and readout schemes using a microprocessor controlled,temperature-stabilized driver can be used to determine resistance fromdrive current and drive voltage readings. That information shows thatincidental resistance (temperature coefficient in leads and packages andshunt resistors, for instance) do not overwhelm the small resistancechanges used as a measurement parameter.

For embodiments using a second detector for reference, the devices canbe configured in a balanced bridge. Referring to FIG. 13, a Wheatstonebridge drive circuit 160 is shown. The Wheatstone bridge is astraightforward analog control circuit used to perform the function ofmeasuring small resistance changes in a detector. It is very simple,very accurate, quite insensitive to power supply variations andrelatively insensitive to temperature. The circuit is “resistor”programmable but depends for stability on matching the ratio ofresistors. In one form, an adjacent “blind” detector element—anidentical bolometer element filtered at some different waveband—is usedas the resistor in the other leg of the bridge, allowing compensationfor instrument and component temperatures and providing only adifference signal related to infrared absorption in the target gas.

In some embodiments, a wavelength selective emission device can beoperated as both a source and a detector. For example, the emissiondevice is heated using a thermal source, such as a resistive filamentexcited by an electrical current. The infrared radiation excites thearrangement of surface elements establishing a resonant coupling of thesurface elements to other surface elements and to the ground plane. Theresult is an IR emission having a preferred spectra width (e.g.,narrowband or wideband, depending upon the selection of designparameters). Heat is then removed from the source and the emissiondevice is allowed to cool. The device can be used as a bolometer alsodetecting IR from an external environment or its own self-emission. Theminimum duration of time between heating and cooling is limited by thethermal relaxation of the emission device. Preferably the thin filmdevice is extremely thin, on the order of 10 μm or less, providing avery low thermal mass. Such thin film devices are capable of rapidcooling and can support thermal cycles approaching 1 to 2 Hz or evengreater.

Referring to FIG. 14A, one embodiment of a target material detector 85provides an IR source including wavelength selective emission device 87as described herein. Thus, the emission device 87 emits IR radiation ata wavelength selected to coincide with an absorption band of a targetmaterial, such as a gas. The resonant emission device 87 is aligned toemit radiation toward a target material (e.g., a gas). A reflectingsurface such as a retro-reflective mirror, or a spherical mirror 84, ispositioned opposite the emission device 87 (e.g., at a radial center ofthe spherical mirror), leaving a channel therebetween to accommodate asample of the gas to be inspected for presence of the target component.In operation, radiation emitted from the emission device 87 passesthrough the gas sample toward the mirror 84. That portion of emittedradiation not absorbed by the sample gas reflects off of the mirror 84and travels back toward the emission device 87 traversing the sample gasonce again. When configured to act as an absorber and a receiver, theemissive device 87 detects the amount of received energy at the resonantwavelength. The detected value can be compared to the emitted value todetermine an absorption value indicative of the target gas.

When a wavelength selective surface having multiple resonances is used,each of the multiple resonances can be individually tuned to arespective one of more than one target components. Such a device 85 iscapable of detecting a preferred combination of different targetelements. When all of the two or more target elements are present,absorption of the multi-resonant emissions result in a minimum detectedreturn, as all of the multiple resonant emissions will endureabsorption. However, when one or more of the two or more target elementsare absent from the mixture, at least one of the corresponding resonantradiation emissions will suffer little or no absorption yielding anon-minimum detected return.

In some embodiments, a second emission device 86 is provided in thevicinity of the first 87. The first emission device 87 is tuned to thegas, while the second emission device 86 is tuned to a differentwavelength, chosen to be outside the absorption band of any targetelements in the gas. The return from the second emission device 86 canbe used to measure other effects, such as ambient temperature changesand long-term changes due to device degradation. Results from the secondemission device 86 can be combined with results from the first device87, using techniques described herein, to effectively remove thesesecondary effects.

Referring to FIG. 14B, another embodiment of a reflective gas sensor 85′using a separate emission device 87′ and detection device 86′. A mirror84′ is disposed within the optical path between the emission device 87′and the detection device 86′. The sample material is also disposedbetween the optical path, such that emitted radiation traverses thesample, such that absorption by a target element will bet evident by areduced return at the detector 86′.

In some embodiments, at least one of the layers of a wavelengthselective device provides a controllable electrical conductivity.Preferably, the conductivity of the associated layer can be controlledusing an external control mechanism to alter the resonant performance ofthe wavelength selective device. Referring now to FIG. 15A, a wavelengthselective device 200 includes an arrangement of conductive surfaceelements 202 disposed above a ground layer 204. The conductive surfaceelements 202 are isolated from each other and separated from the groundlayer 204 by an intermediate insulating layer 206. The wavelengthselective device 200 provides a resonant response to incidentelectromagnetic radiation that depends on one or more of the designfeatures of the device 200 as described herein. In the presence ofelectromagnetic radiation at wavelengths in and around the one or moreresonant peaks, electromagnetic coupling fields are produced in andaround the conductive surface elements 202 and particularly within theinsulating layer 206 between each of the elements 202 and a localizedregion of the ground layer 204.

In the exemplary embodiment, an over layer 208 of insulating materialcovers the surface elements 202. In particular, the over layer 208 ismade from a material having an electrical conductivity value that can bealtered by an external control mechanism. When controlled to have afirst conductivity that is substantially insulating, the device 200demonstrates a resonant response to one or more of reflectivity,absorption, and emissivity. The first conductivity can be said toprovide a relatively high impedance value that sufficiently maintainselectrical isolation of the conductive surface elements 202. Uponactivation by the external control mechanism, the over layer 208provides a second conductivity value that is non-insulating, orelectrically conducting. Being electrically conductive, or having arelatively low impedance value, the over layer 208 changes the resonantresponse of the device 200.

In some embodiments, the over layer 208 includes a semiconductor, suchas silicon. The semiconductor itself behaves as an insulator. When dopedwith an appropriate element, the semiconductor can become electricallyconductive in the presence of an applied electric field. Such techniquesare well known to those skilled in the art of semiconductor fabrication.In order to provide an electric field to the semiconductor material, atleast two terminals are provided: a source terminal 210 and a drainterminal 212. The intermediate insulating layer 206 can include anoxide, and the electrically conducting metal ground plane 204 can beused as a gate terminal, such that the device represents ametal-oxide-semiconductor (MOS) field effect transistor (FET). Inparticular, the structure represents a form of transistor referred to asa thin-film transistor (TFT).

Upon application of a sufficient gate-to-source voltage (Vgs), theelectrical conductivity of the semiconductor over layer 208 changes frominsulating (off) to conducting (on). Being electrically conducting, thesurface elements 202 are short circuited together. Such a substantialchange to the structure quenches the electromagnetic fields previouslyestablished between the surface elements 202 and the ground layer 204,thereby change the resonant response. When the surface elements 202 areshorted together in this manner, the resonant response essentiallydisappears, such that the wavelength selective device 200 can beselectively turned on and off as desired by controlling voltage signalapplied between the gate and source terminals. This can be used tomodulate the resonant response, be it reflectivity, absorption, andemissivity, at speeds (e.g., kilohertz through megahertz, and higher)much faster than would otherwise be possible considering the thermalrelaxation response of the device. Thus, the resonant response is nolonger limited by a thermal relaxation between cycles.

In other embodiments, the device 200 includes a similar architecturewith an over layer 208 formed from an optically responsive material,such as photovoltaic material. Without illumination, or withinsufficient illumination below some threshold value, the photovoltaicmaterial 208 is substantially insulating allowing the device 200 toexhibit a resonant response according to the design parameters of thedevice 200. When illuminated sufficiently, the conductivity of the overlayer 208 changes, becoming non-insulating, or electrically conductive.Such an increase in electrical conductivity substantially changes theresonant behavior of the device 200 by altering, and in some instances,electrically short-circuiting the arrangement surface elements 202.Thus, resonant performance of the device at one or more wavelengths ofinterest can be substantially modified by application of light energy atthe same or different wavelengths. In such an embodiment, there would beno need for either a source terminal 210 or a drain terminal 212.

Referring to FIG. 15B, a top perspective view of one such device 220 isshown having an arrangement of surface elements 222 disposed on aninsulating intermediate layer 224. A ground layer 226 is providedbeneath the intermediate layer 224. An over layer 227 is applied overthe arrangement of surface elements 222, having source terminal 223 anda drain terminal 225 disposed along opposite ends of the over layer 227.The entire device can be formed on a substrate 228. In some embodimentssubstrate 228 can be rigid, such as on a base Si wafer providing supportto the transistor structure 220. In other embodiments, the substrate 228can be flexible so that the device 220 can be contoured to the surfaceon which it is applied. At least one suitable flexible substrateincludes polyimide films, commercially available from DuPont under thetrade name KAPTON. Electrical contact can be made from an externalsource to one or more of the gate 226, source 223, and drain 225terminals, such that application of an applied electrical signal canalter the conductivity of the over layer 227, thereby changing theresonant response of the wavelength selective device 220.

More generally, a similar approach can be used to controllably vary theconductivity of any one of the layers of a multi-layer wavelengthselective device. In one embodiment, a ground plane layer can beincluded having a controllable conductivity. In some embodiments, theconductivity can be controlled by the application of an electricalsignal. For example, the ground layer can include a suitably dopedsemiconductor material supporting an electrical current in the presenceof an electric field above a threshold value. Thus, in the presence of asufficient electric field, the ground layer becomes electricallyconducting and the wavelength selective device operates according to theprincipals of the invention yielding a resonant response according tothe chosen design parameters. However, upon variation of the electricfield below the threshold, or its removal altogether, the ground layerbecomes non-conducting, effectively removing the ground layer from thedevice. Such a substantial change in the configuration of the devicequenches the standing wave electric fields in the dielectric and changesthe overall reflection or absorption/emission resonance.

In another embodiment, the insulating layer includes a controllableconductivity. For example, the conductivity can be controlled by anelectrical signal using a device such as a semiconductor for theinsulating layer. Without application of a sufficient controllingelectrical field, the insulating layer remains insulating allowing thewavelength selective device to operate according to the principals ofthe present invention yielding and providing a resonant responseaccording to the chosen design parameters. However, upon the applicationof a sufficient electrical field, the insulating layer changes frominsulating to non-insulating (or semi-insulating), thereby quenching theelectromagnetic fields in the intermediate layer. Such a substantialchange in the behavior of the ground layer alters the resonantperformance, essentially turning the resonant performance off.

In addition to semiconductors, other materials can be used to provide aelectrical conductivity controllable by an external control signal.Other examples include photovoltaic materials as described above andthermally responsive materials, such as pyroelectric materials thatchange conductivity in response to heat. Still other examples includechemically responsive materials, such as polymers that changeconductivity in response to a local chemical environment. For example,the wavelength selective device includes an intermediate insulatinglayer formed from a photoconductor with a conductivity modified byincident light. Such a device would have an infrared reflection, andemission spectrum that could be modified by an external light source.

Alternatively or in addition, the intermediate layer includes adielectric layer having an electrical conductivity that changes inresponse to its local chemical and/or physical environment. Such adevice can serve as a remote sensor or tag for the relevant chemical orphysical changes. Such a device can be remotely monitored through itsinfrared reflection/emission signature.

In yet other embodiments, the intermediate dielectric layer can have aconductivity or index of refraction that can be modified by acombination of the local environment and external illumination. One suchexample includes a fluorescent polymer.

Any of the above controllable devices can be used as an externallymodulated, tuned electromagnetic emitter. This is particularlyadvantageous in the infrared band, wherein the device can be modulatedrapidly, and faster than would otherwise be possible in view of thermalrelaxation of the material.

A wavelength selective device that selectively reflects and/or emitselectromagnetic radiation of a preferred wavelength can be used as apicture element, or pixel in a display device. Referring to FIG. 16, apixel 300 is shown including a two-by-two rectangular matrix ofsub-pixel elements 302 a, 302 b, 302 c, 302 d (generally 302). A pair ofcolumn electrodes 304 a, 304 b (generally 304) is aligned vertically,with each column electrode 304 connected to both sub-pixels 203 in itsrespective column. Likewise, a pair of row electrodes 306 a, 306 b(generally 306) is aligned horizontally, with each row electrode 306connected to both sub-pixels 203 in its respective row. In particular,each of the sub-pixels can be individually addressed by applying asignal to the singular combination of column and row electrodes 304, 306interconnected to the addressed sub-pixel 302. The pixel 300 can beformed on a substrate using techniques known to those skilled in the artof thin film displays, in which the film pixel elements include aresonant reflectivity and/or emissivity response as described herein.

A schematic representation of a matrix display is shown in FIG. 17,using an array of pixel 300 elements according to principles of thepresent invention. In some embodiments, each of the sub-pixels 302provides a resonant response at a substantially equivalent wavelength,or at least within the same band (e.g., the same IR band). In someembodiments, the intensity of the reflective response can be variedaccording to an applied control signal of each sub pixel 302. Suchvariation can be used to vary the intensity of a reflectivity dip(absorption spike) without substantially changing its resonantwavelength. For emissivity applications, such variation of a controlinput can be used to vary the intensity of emission spike, withoutsubstantially changing its resonant wavelength. With variations inintensity, the display 310 can be compared to a black and white visualdisplay, having an array of pixels each displaying a controllable shadeof gray (i.e., intensity).

In other embodiments, the pixel 300 includes an array of sub-pixels 302in which each sub-pixel is tuned to a different respective wavelength.Thus, alternatively or in addition to the ability to control intensityof each of the sub pixels 302 as described above, each of the sub-pixels302 can be actuated to provide a variable intensity, variable wavelengthresponse. With variations in intensity and wavelength, the display 310can be compared to a color visual display, having an array of pixelseach including an array of sub-pixels to display different colors andintensity.

Thus, a complex picture can be formed within a portion of theelectromagnetic spectrum determined by the resonant wavelength (e.g.,IR), using a matrix display formed from a matrix of wavelength selectivedevice as described using the principles described herein. The matrixdisplay 310 can operate in a reflection mode, in which the display 310is illuminated by an external electromagnetic radiation (e.g., anexternal IR source). A detector receiving reflections from the matrixdisplay 310 captures a two-dimensional image formed thereon by selectiveactivation of the individual pixels 300 of the array 310.

Alternatively or in addition, the matrix display 310 can operate in anemission mode, in which the display 310 emits electromagnetic radiation(e.g., IR). A detector, without the need of an external IR source,receives emissions from the matrix display 310, capturing an imageformed thereon through selective activation of the individual pixels 300of the array 310.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A tunable device for selectively coupling electromagnetic radiationcomprising: a first electrically conductive layer including a pluralityof discrete surface elements formed on or in the layer; an electricallyinsulating intermediate layer defining a first surface in communicationwith the electrically conductive surface layer; a second, continuouselectrically conductive layer in communication with a second surface ofthe electrically insulating intermediate layer; and an electrode inelectrical communication with at least one of the first electricallyconductive layer, the electrically insulating intermediate layer, andthe second continuous, electrically conductive layer, the plurality ofdiscrete surface elements resonantly coupling at least a portion of theelectromagnetic radiation with respect to the continuous electricallyconductive layer; wherein the first electrically conductive layer, theelectrically insulating intermediate layer, and the second continuous,electrically conductive layer form a wavelength selective element havingat least one reflection or absorption-emission band; and wherein thewavelength selective element comprises a material having a materialproperty that is variable in response to an external signal applied tothe device, and wherein variation of the material property tunes the atleast one reflection or absorption-emission band.
 2. The device of claim1, wherein the plurality of discrete surface elements comprises an arrayof uniform-sized electrically conductive elements.
 3. The device ofclaim 2, wherein the uniformly shaped elements are selected from thegroup consisting of: closed curves; ellipses; circles; rectangles;squares; polygons; triangles; hexagons; parallelograms; stars having atleast three legs; annular structures; and combinations thereof.
 4. Thedevice of claim 1, wherein the plurality of discrete surface elementsare arranged in an array.
 5. The device of claim 4, wherein the array isselected from the group consisting of: rectangular grids; square grids;triangular grids; Archimedean grids; and random arrangements.
 6. Thedevice of claim 1, wherein at least one of the first and secondelectrically conductive layers is formed from a metal.
 7. The device ofclaim 1, wherein the electrically insulating intermediate layercomprises a dielectric material.
 8. The device of claim 1, wherein atleast one of the first electrically conductive layer, the electricallyinsulating intermediate layer, and the second continuous electricallyconductive layer comprises a semiconductor.
 9. The device of claim 1,wherein the plurality of discrete surface elements of the firstelectrically conductive layer comprises a plurality of through holesdefined in an electrically conductive surface layer.
 10. The device ofclaim 9, wherein the plurality of discrete through holes comprises anarray of uniform-sized elements.
 11. The device of claim 1, wherein thesecond, continuous electrically conductive layer comprises anelectrically activated thermal source in communication with theelectrode, the electrode receiving an electrical input to activate thethermal source.
 12. The device of claim 1, wherein the second,continuous electrically conductive layer comprises a thermistor inthermal communication with the plurality of discrete surface elementsand in communication with the electrode, the electrode enabling remotemonitoring of the thermistor.
 13. The device of claim 1, wherein two ormore of the first electrically conductive layer, the electricallyinsulating intermediate layer, and the second continuous, electricallyconductive layer are configured to provide a controllable switch, theelectrode receiving an electrical input for controlling the switch. 14.The device of claim 1, wherein the external signal comprises at leastone selected from the list consisting of: an electrical signal, achemical signal, an optical signal, and a thermal signal.
 15. The deviceof claim 1, wherein the material property comprises conductivity.
 16. Atunable infrared emitter comprising: a first electrically conductivelayer including a plurality of discrete surface elements formed on or inthe layer; an electrically insulating intermediate layer defining afirst surface in communication with the electrically conductive surfacelayer; a second, continuous electrically conductive layer incommunication with a second surface of the electrically insulatingintermediate layer; and an infrared source in thermal communication withat least one of the first electrically conductive layer, theelectrically insulating layer and the second, continuous electricallyconductive layer, the infrared source generating broadband infraredradiation, the plurality of discrete surface elementselectromagnetically coupling at least a portion of the broadbandinfrared radiation, wherein the first electrically conductive layer, theelectrically insulating intermediate layer, and the second continuous,electrically conductive layer form a wavelength selective element havingat least one infrared reflection or absorption-emission band; andwherein the wavelength selective element comprises a material having amaterial property that is variable in response to an external signalapplied to the device, and wherein variation of the material propertytunes the at least one reflection or absorption-emission band; andwherein the wavelength selective element interacts with the broadbandinfrared radiation to produce a narrowband infrared emission.
 17. Thedevice of claim 16, wherein tuned, narrowband infrared emission has aselectivity of not more than about 0.1, determined at afull-width-half-maximum spectral width.
 18. The device of claim 16,wherein the infrared source comprises an electrical filament.
 19. Thedevice of claim 18, wherein the electrical filament includes the second,continuous electrically conductive layer.
 20. The device of claim 16,wherein a total thickness of the first electrically conductive layer,the electrically insulating layer and the second, continuouselectrically conductive layer is not more than about 10 μm.
 21. Thedevice of claim 20, wherein the device is suspended above a surface. 22.The device of claim 16, further comprising a thermistor in thermalcommunication with the plurality of discrete surface elements, thedevice selectively emitting infrared radiation in a tuned banddetermined by the electromagnetic coupling of the plurality of discretesurface elements.
 23. The device of claim 16, wherein the externalsignal comprises at least one selected from the list consisting of: anelectrical signal, a chemical signal, an optical signal, and a thermalsignal.
 24. The device of claim 16, wherein the material propertycomprises conductivity.
 25. A controllable wavelength selective device,comprising: a first electrically conductive layer including a pluralityof discrete surface elements formed on or in the layer; an electricallyinsulating intermediate layer defining a first surface in communicationwith the electrically conductive surface layer; a second, continuouselectrically conductive layer in communication with a second surface ofthe electrically insulating intermediate layer, wherein at least one ofthe first electrically conductive layer; the electrically insulatingintermediate layer; and the second, continuous electrically conductivelayer comprises a material having an electrical conductivity which iscontrolled in response to an external signal applied to the device;wherein the first electrically conductive layer, the electricallyinsulating intermediate layer, and the second continuous, electricallyconductive layer form a wavelength selective element having at least onereflection or absorption-emission band; and wherein a property of the atleast one reflection or absorption-emission band is controlled inresponse to the conductivity of the material.
 26. The device of claim25, wherein at least one of the first electrically conductive layer andthe second, continuous electrically conductive layer comprises anelectrical conductivity controllable between conducting andnon-conducting, the controlled conductivity usable to control resonantperformance of the device.
 27. The device of claim 26, wherein at leastone of the first electrically conductive layer and the second,continuous electrically conductive layer comprises a semiconductormaterial having a controllable electrical conductivity responsive to anelectrical input.
 28. The device of claim 26, wherein at least one ofthe first electrically conductive layer and the second, continuouselectrically conductive layer comprises a pyroelectric material having acontrollable electrical conductivity responsive to a thermal input. 29.The device of claim 25, wherein the electrically insulating intermediatelayer comprises an electrical conductivity controllable betweeninsulating and non-insulating, the controlled conductivity usable tocontrol resonant performance of the device.
 30. The device of claim 29,wherein the electrically insulating intermediate layer comprises asemiconductor material having a controllable electrical conductivityresponsive to an electrical input.
 31. The device of claim 29, whereinthe electrically insulating intermediate layer comprises a pyroelectricmaterial having a controllable electrical conductivity responsive to athermal input.
 32. The device of claim 29, wherein the electricallyinsulating intermediate layer comprises an optically responsive materialhaving a controllable electrical conductivity responsive to an opticalinput.
 33. The device of claim 29, wherein the electrically insulatingintermediate layer comprises a chemically responsive material having acontrollable electrical conductivity responsive to a change in a localchemical environment.
 34. The device of claim 25, wherein the externalsignal comprises at least one selected from the list consisting of: anelectrical signal, a chemical signal, an optical signal, and a thermalsignal.
 35. A method of manufacturing a wavelength selective devicecomprising: forming a continuous, electrically thin conductive groundlayer on a substrate; applying an electrically thin insulating layer toa top surface of the ground layer; forming an electrically thin outerconductive layer on the electrically thin insulating layer, forming aplurality of discrete surface elements on or in the outer conductivelayer; forming a wavelength selective element having at least oneinfrared reflection or absorption-emission band comprising theelectrically thin conductive ground layer, the electrically thininsulating layer, and the electrically thin outer conductive layer; andwherein forming the wavelength selective surface comprises forming thewavelength selective surface to comprise a material having a materialproperty that is variable in response to an external signal applied tothe device, wherein variation of the material property tunes the atleast one reflection or absorption-emission band.
 36. The method ofclaim 35, wherein at least one of the layers is deposited using a thinfilm deposition technique.
 37. The method of claim 35, wherein thediscrete surface elements are formed using at least one of: laserablation, removing selected regions of the conductive material from thesurface; nano-imprinting; stamping; and standard semiconductorfabrication techniques.
 38. The method of claim 35, wherein thesubstrate is flexible.
 39. The method of claim 35, further comprisingremoving a sacrificial layer between a bottom surface of the groundlayer and a substrate, removal of the sacrificial layer releasing amulti-layered thin-film device.
 40. The method of claim 39, wherein thethickness of the multi-layered thin-film device is not more than about10 μm.
 41. The method of claim 35, wherein the external signal comprisesat least one selected from the list consisting of: an electrical signal,a chemical signal, an optical signal, and a thermal signal.
 42. Themethod of claim 35, wherein the material property comprisesconductivity.